Bacillus sphaericus strain 1593 has been proven effective as a microbial insecticide for control of larvae of several species of disease-vectoring mosquitoes. This bacterium kills mosquito larvae by means of a protein toxin associated with sporulating cells and mature spores. The usefulness of this organism as a biocontrol agent is enhanced by its abililty to recycle and persist in nature for long periods of time. Recently, a plasmid transformation system has been developed for this organism which will be used to manipulate the genes effecting insecticidal toxin production, toxicity, and host range. Techniques fundamental to recombinant DNA methodology will be adapted for use with this bacterium in order to accomplish these genetic modifications. The genetic information specifying the extremely potent larvicidal protein produced by a related bacterium, Basillus thuringiensis var. israelensis, will be cloned in Bacillus subtilis and ultimately transferred into B. sphaericus strain 1593.